Since the attempted shoe-bombing of a US commercial aircraft shortly after the attacks of Sep. 11, 2001, there has been a concerted effort to develop automated systems capable of detecting explosive devices concealed in footwear worn by airline passengers. While some progress has been made towards this goal, a reliable detection method with a high probability of detection and a low false alarm rate has remained elusive.
Currently, most airports implement the requirement to inspect footwear by having passengers remove their shoes before passing them through the x-ray inspection machines used to inspect carry-on baggage. The relatively high-resolution dual-energy images from these systems are effective at allowing an operator to detect any concealed explosives and associated wires and detonators. Unfortunately, however, the extra time involved with passengers removing, and subsequently reclaiming, their shoes significantly slows down the throughput of the security checkpoints. It therefore remains a top priority of organizations responsible for aviation security to come up with methods for inspecting shoes that do not require that passengers remove their footwear.
While transmission imaging has been practiced since the time of Röntgen, it has never been practical to obtain useful information with respect to a person's footwear while the person was walking—nor was the concept ever suggested. This is largely because the direct-conversion detector arrays required to produce very high resolution images with the required high readout rates and 100% detection efficiency (so as to enable an extremely low radiation dose to the foot) have not, until recently, been available at a cost that makes such a concept practical.
Knees have been imaged in motion in x-ray transmission for motion studies (University of Florida) using two-dimensional areal arrays of detectors that typically use scintillator material to convert the incident x-ray energy into light, which is subsequently detected by photodetectors such as photodiodes. This is a very inefficient process, and more x-rays must therefore be incident on the imaged object to create an image that is comparable to that obtained with a solid-state, direct conversion detector array, in which each x-ray is detected directly in the semiconductor material with 100% efficiency. Moreover, scintillator based areal arrays typically operate in current integration mode, rather than in the more effective photon-counting mode of the solid-state arrays. The scintillator-based areal arrays therefore result in a much larger dose to an imaged object than would be acceptable in an application such as scanning the feet of travelers. In addition, areal arrays are typically too costly for security applications and are also subject to image degradation due to the detection of scattered x-rays. Finally, no scheme of post-collimators has yet proven practical with a highly segmented areal array, yet, post-collimation would be necessary to prevent the detection of scattered radiation.
Detection of concealed explosives in shoes presents peculiar resolution requirements, and deployment in an airport context where travelers are subject to cumulative dose limitations, imposes limitations on the spectrum and flux available. Moreover, the integration time is limited by the requisite passenger throughput to about 1-2 ms per image line in order to achieve a useful image resolution. Therefore, it has been believed to date that the only practical way to detect explosives in shoes is either by removing the shoes and passing them through a regular baggage x-ray screening device or by the detection of explosive materials by chemical trace detection using mass spectroscopy and/or gas chromatography techniques, or by bulk detection techniques such as Nuclear Quadrupole Resonance (NQR) or electro-magnetic dielectric analysis.